A conventional sewn air bag is manufactured by sewing a plurality of given shaped cloths cut out of a woven fabric. It has been known that a hollow weave air bag can be made of a hollow weave fabric (so-called double weave fabric) through a manufacturing process in which the sewing work step for the circumferential seam formation can be eliminated. With elimination of the sewing work step, the proposed hollow weave air bag can be advantageously made without problems of the product reliability arising from manual sewing work in a conventional sewing made air bag manufacturing process, and can be supplied at a reduced price such that air bag apparatus is carried in wide variety of cars.
In Japanese Unexamined Patent Publication (Kokai) No. 1-254446 and U.S. Pat. No. 5,336,538, there are described coated air bags of a circular hollow weave fabric in which the circumferential periphery of the hollow portion is surrounded by a bind weave.
Air bags made of an bare fabric with no polymeric rubber coating have been developed with an attempt to obtain a light weight air bag which can be folded into a minimized shape.
In the patent specifications of Japanese Unexamined Patent Publication (Tokuhyo 4-504988 and WO 90/09295, uncoated circular hollow weave air bags made from a hollow weave fabric are described. In a currently known uncoated air bag, the fabric material is made in part more gas permeable so as to obtain a filtering effect for the discharge of inflating gas in the air bag, and the weave construction of the part is woven either in a lower weave density or in a more open weave such as a satin weave, twill weave and basket weave which have a more open weave construction.
The fabric material woven in a more open weave construction can assure the gas dischargeability required for an air bag. There are, however, some hazards encountered with use of a open weave construction; the vehicle occupant may suffer burns from the hot inflating gas which passes through an opened weave structure, or suffer eye injury due to the scattering residues contained in the hot gas. Since a base fabric of an open weave construction can be easily damaged by slippage of the weave structure, an air bag made from an open weave construction can be easily broken at its circumferential joined portion and the reinforced periphery edge of the opening for attachment of the gas generator when the air bag is deployed.
In Japanese Unexamined Patent Publication (Kokai) No. 3-128743 and U.S. Pat. No. 5,114,180, there are described examples of air bags in which the outer circumferential joined portion as well as the inner periphery portion around the hole for attachment of a gas generator are coated so that the afore-mentioned drawback can be eliminated. This method, however, needs an application of coating work and cannot make a light air bag with a soft hand in practice.
In Japanese Unexamined Patent Publication (Kokai) No. 2-158442, there is described a one piece air bag made of a hollow weave fabric in which the top side fabric facing the vehicle occupant and the bottom side fabric of the air bag have their respective own specific gas permeability range, qA and qB. However, the ratio of their gas permeability, qB/qA is greater than 2.5 which consequently may lead the air bag to fail to absorb of the impact energy of the vehicle occupant in the event of collision.
Furthermore, the circumferential joining portion of a hollow weave air bag is formed by two pieces of fabrics joined together by the bind weave (so called stitch weave) construction. Accordingly, the weave density in the bind weave is equal to the total number of woven yarns of the fabrics. As a result, the joining portion has such a very coarse and dense weave construction that could injure the vehicle occupant by abrasion when the air bag deploys at a very high speed. Because of this danger, the manufacture of air bags from a hollow weave fabric necessitates the turning inside out work step for making the joined potion to be placed inside of the bag body to obtain a ready to use form of the air bag. Naturally, the turning inside-out work is a factor raising the manufacturing cost of a hollow weave air bag.